[POWERPC] Check for NULL ppc_md.init_IRQ() before calling

Check to make sure ppc_md.init_IRQ has been set before calling it.
Signed-off-by: NSonny Rao <sonny@burdell.org>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/irq.c

@@ -336,7 +336,8 @@ void do_IRQ(struct pt_regs *regs)
 
 void __init init_IRQ(void)
 {
-	ppc_md.init_IRQ();
+	if (ppc_md.init_IRQ)
+		ppc_md.init_IRQ();
 #ifdef CONFIG_PPC64
 	irq_ctx_init();
 #endif